Boris Johnson says he is willing to give the EU the “commitments” it needs that alternatives to the Northern Ireland border backstop can be found and put into effect.

In a new development in the Brexit negotiations, the prime minister’s letter to EU Council President Donald Tusk will be seen as an attempt to convince Europe’s leaders that he is willing to provide guarantees that other arrangements will be viable.

He wrote: “The backstop should be replaced with a commitment to put in place [alternative arrangements] as far as possible before the end of the transition period, as part of the future relationship.

“I recognise that there will need to be a degree of confidence about what would happen if these arrangements were not all fully in place at the end of [the transition] period.

“We are ready to look constructively and flexibly at what commitments might help.”

“Alternative arrangements” to the backstop have long been touted by supporters of Brexit as a potential solution to the Northern Ireland border problem. They include livestock checks away from the border and a “trusted trader” programme for goods.

In the same letter he challenged the EU to make a legal promise not to impose a hard border. The prime minister wrote: “We would be happy to accept a legally binding commitment to this effect and hope the EU would do likewise.”

He also reiterated his opposition to the backstop – which would force the UK to obey some EU rules if the border between Ireland and Northern Ireland could not be kept open.

He wrote that the backstop is “simply unviable” and “anti-democractic and inconsistent with the sovereignty of the UK as a state”.